The present invention relates generally to an improved method of forming compacted pellets from powders comprising moisture-sensitive organic materials wherein moisture is removed from such powder and using such pellets in physical vapor deposition to make an organic layer on a structure which will form part of an OLED.
An organic light-emitting device (OLED), also referred to as an organic electroluminescent device, can be constructed by sandwiching two or more organic layers between first and second electrodes.
In a passive-matrix OLED of conventional construction, a plurality of laterally spaced light-transmissive anodes, for example indium-tin-oxide (ITO) anodes, are formed as first electrodes on a light-transmissive substrate such as, for example, a glass substrate. Two or more organic layers are then formed successively by physical vapor deposition of respective organic materials from respective sources, within a chamber held at reduced pressure, typically less than 10xe2x88x923 Torr. A plurality of laterally spaced cathodes is deposited as second electrodes over an uppermost one of the organic layers. The cathodes are oriented at an angle, typically at a right angle, with respect to the anodes.
Such conventional passive-matrix OLEDs are operated by applying an electrical potential (also referred to as a drive voltage) between appropriate columns (anodes) and, sequentially, each row (cathode). When a cathode is biased negatively with respect to an anode, light is emitted from a pixel defined by an overlap area of the cathode and the anode, and emitted light reaches an observer through the anode and the substrate.
In an active-matrix OLED, an array of anodes are provided as first electrodes by thin-film transistors (TFTs) which are connected to a respective light-transmissive portion. Two or more organic layers are formed successively by vapor deposition in a manner substantially equivalent to the construction of the aforementioned passive matrix device. A common cathode is deposited as a second electrode over an uppermost one of the organic layers. The construction and function of an active matrix organic light-emitting device is described in commonly-assigned U.S. Pat. No. 5,550,066, the disclosure of which is herein incorporated by reference.
Organic materials, thicknesses of vapor-deposited organic layers, and layer configurations, useful in constructing an organic light-emitting device, are described, for example, in commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; and 4,769,292, the disclosures of which are herein incorporated by reference.
Organic materials useful in making OLEDs, for example organic hole-transporting materials, organic light-emitting materials predoped with an organic dopant, and organic electron-transporting materials can have relatively complex molecular structures with relatively weak molecular bonding forces, so that care must be taken to avoid decomposition of the organic material(s) during physical vapor deposition.
The aforementioned organic materials are synthesized to a relatively high degree of purity, and are provided in the form of powders, flakes, or granules. Such powders or flakes have been used heretofore for placement into a physical vapor deposition source wherein heat is applied for forming a vapor by sublimation or vaporization of the organic material, the vapor condensing on a structure to provide an organic layer thereon.
Several problems have been observed in using organic powders, flakes, or granules in physical vapor deposition:
(i) powders, flakes, or granules are difficult to handle because they can acquire electrostatic charges via a process referred to as triboelectric charging;
(ii) powders, flakes, or granules of organic materials generally have a relatively low physical density (expressed in terms of weight per unit volume) in a range from about 0.05 to about 0.2 g/cm3, compared to a physical density of an idealized solid organic material of about 1 g/cm3;
(iii) powders, flakes, or granules of organic materials have an undesirably low thermal conductivity, particularly when placed in a physical vapor deposition source which is disposed in a chamber evacuated to a reduced pressure as low as 10xe2x88x926 Torr. Consequently, powder particles, flakes, or granules are heated only by radiative heating from a heated source, and by conductive heating of particles or flakes directly in contact with heated surfaces of the source. Powder particles, flakes, or granules which are not in contact with heated surfaces of the source are not effectively heated by conductive heating due to a relatively low particle-to-particle contact area; and
(iv) powders, flakes, or granules can have a relatively high ratio of surface area/volume of such particles and a correspondingly high propensity to entrap air and/or moisture between particles under ambient conditions. Consequently, a charge of organic powders, flakes, or granules loaded into a physical vapor deposition source which is disposed in a chamber must be thoroughly outgased by preheating the source once the chamber has been evacuated to a reduced pressure. If outgasing is omitted or is incomplete, particles can be ejected from the source together with a vapor stream during physical vapor-depositing an organic layer on a structure. An OLED, having multiple organic layers, can be or can become functionally inoperative if such layers include particles or particulates.
Each one, or a combination, of the aforementioned aspects of organic powders, flakes, or granules can lead to nonuniform heating of such organic materials in physical vapor deposition sources with attendant spatially nonuniform sublimation or vaporization of organic material and can, therefore, result in potentially nonuniform vapor-deposited organic layers formed on a structure.
It is an object of the present invention to provide a method of compacting moisture-sensitive organic material adaptable for making an organic layer on a structure which will form part of OLED.
It is another object of the present invention to provide a method of compacting moisture-sensitive organic powder into a solid pellet.
It is a further object of the invention to provide a method of making an organic layer from a consolidated pellet of organic material and on a structure which will form part of an OLED.
In one aspect, the present invention provides an improved method of compacting moisture-sensitive organic material adaptable for making an organic layer on a structure, which will form part of an organic light-emitting device, comprising the steps of:
(a) placing such a desiccant material in a powder form inside a die cavity and applying a pressure to such a desiccant powder in the die cavity sufficient to compact into a porous desiccant bed;
(b) providing moisture-sensitive organic material in a powder form; and
(c) placing such moisture-sensitive organic material inside the die cavity over the porous desiccant bed, and applying sufficient heat to the moisture-sensitive organic material in the die cavity to cause moisture to escape from the moisture-sensitive organic material and be absorbed by the porous desiccant bed, and then applying sufficient pressure to compact the moisture-sensitive organic material into a solid organic pellet.
In another aspect, the present invention provides a method of compacting moisture-sensitive organic material adaptable for making an organic layer on a structure, which will form part of an organic light-emitting device, comprising the steps of:
(a) providing a desiccant material in a powder form;
(b) placing a first load of desiccant powder inside a die cavity over a second punch and moving a first punch to apply pressure to such desiccant powder in the die cavity to compact into a first porous desiccant bed;
(c) placing moisture-sensitive organic material in a powder form inside the die cavity over the first porous desiccant bed, and applying sufficient pressure with the first punch to level the top surface of the moisture-sensitive organic material;
(d) placing a second load of desiccant powder over the level surface of moisture-sensitive organic material and apply sufficient pressure with the first punch to form a second porous desiccant bed; and
(e) applying sufficient heat to the moisture-sensitive organic material inside the die cavity to cause moisture to escape from the moisture-sensitive organic material and be absorbed by the first and second porous desiccant beds, and then applying sufficient pressure to compact the moisture-sensitive organic material into a solid organic pellet.
Another feature of the method described above is providing a porous punch with means to pass dry inert gas through the organic material in powder form placed inside the die cavity.
In another aspect, the present invention provides a method of making an organic layer from an organic material on a structure, which will form part of an OLED comprising the steps of:
(a) providing a moisture-free solid organic pellet comprising at least one organic host and one organic dopant;
(b) placing such an organic solid pellet inside a heating source disposed in a physical vapor deposition chamber;
(c) positioning the substrate in the chamber and in a spaced relationship with respect to the heating source;
(d) evacuating the chamber to a reduced pressure; and
(e) applying heat to the source to cause at least a portion of the solid organic pellet to sublime to provide a vapor of the organic materials which form the organic layer on the substrate
Another feature of the present invention is that a solid organic pellet can be compacted by the method of the invention wherein at least one organic host material and at least one organic dopant materials are mixed prior to the compaction process.
A feature of the present invention is that the method of compacting moisture-sensitive organic powder into a moisture-free solid pellet can be accomplished with relatively simple tools and at a location remote from a location of use of such pellet in a physical vapor deposition apparatus.
Another feature of the present invention is that the OLED displays made from using moisture-free organic pellets have better electro-optical performance.
Another feature of the present invention is that the method of compacting organic powder into a solid pellet substantially facilitates transfer or shipping of organic material in and between different locations.
Another feature of the present invention is that a plurality of pellets of organic material, prepared by the method of the present invention, can be handled, transferred or shipped in a container having a significantly reduced volume with respect to a container for compacting, transferring or shipping organic material in powder form and of comparable weight.
Another feature of the present invention is that a consolidated pellet of an OLED-material can be made by the method of the present invention wherein a powder of at least one OLED host-material and a powder of at least one organic dopant material are mixed or blended to provide a mixture prior to compacting the mixture into a consolidated pellet.
Another feature of the present invention is that a solid organic pellet comprising a host and one or more organic dopants eliminates the need of co-evaporation which require more than one evaporation source disposed inside a vacuum chamber.